1. Field of the Invention
The present invention relates in general to automatic transmissions, and more particularly to clutches (viz., front clutch, rear clutch, and the like) installed in the transmissions for conducting the gear change operation of the transmissions. More specifically, the present invention is concerned with the clutches of a type which can assuredly suppress air from being left in a piston operating fluid chamber of the clutch upon engagement of the clutch.
2. Description of the Prior Art
As is known, a clutch, such as front clutch, rear clutch or the like, of an automatic transmission comprises generally a clutch drum having a piston operating fluid chamber, a piston axially slidably received in the piston operating fluid chamber, a clutch hub coaxially disposed in the clutch drum, and a clutch plate pack interposed between the clutch drum and the clutch hub having one end directed to the piston. The clutch plate pack includes first and second groups of plates which are alternately juxtaposed. When the fluid chamber is fed with a pressurized operating fluid, the piston is forced to press the clutch plate pack thereby to engage the first and second groups of clutch plates. Under this engaged condition, the clutch drum and the clutch hub are united and thus can rotate together. While, when the pressurized operating fluid is drawn from the fluid chamber, the piston releases the clutch plate pack thereby to cancel the engagement between the first and second groups of clutch plates. Thus, under this disengaged condition, the clutch drum and the clutch hub can rotate separately or individually.
In order to smooth and hasten the change from the engaged condition to the disengaged condition of the clutch plate pack, some of the clutches are equipped with a so-called "air introduction system" which allows introduction of air into the fluid chamber when the pressurized operating fluid is being drawn from the fluid chamber. Of course, the system is equipped with an air discharging means through which the air is discharged from the fluid chamber when the operating fluid is fed to the fluid chamber.
However, due to the inherent construction, some of the clutches having such air introduction systems tend to have a drawback when the operating fluid is fed to the fluid chamber. That is, the air which has entered the fluid chamber can not be smoothly discharged through the air discharging means even when the operating fluid is being fed to the fluid chamber. This phenomenon induces a dull movement of the piston and thus a dull and slow engagement of the clutch plate pack, which tend to cause an undesired clutch slippage phenomenon. Thus, when practically mounted on motor vehicles, the automatic transmissions having such clutches tend to produce a marked shift shock upon gear changing. Of course, the marked shift shock quickens the abrasion of the clutches.